Satellite and Ring Systems

Abstract

After having studied the Earth’s Moon in Chap. 8, we now look at the moons of the other planets of the solar system. A quick look at the two moons of Mars is followed by a detailed examination of the four Galilean satellites of Jupiter: Io, Europa, Ganymede and Callisto. Topics include their surface features, interior composition and structure, and atmosphere. A survey of the intermediate-sized moons of Saturn, including Enceladus and its icy geysers, is followed by an extensive examination of Saturn’s largest moon, Titan: its dense atmosphere, surface features, and methane-ethane lakes. After discussing the moons of Uranus and Neptune, we look at the Pluto-Charon double dwarf planet system. The chapter concludes with a study of the rings of the giant planets, focusing on Saturn’s rings, and interactions between its moons and rings.

Challenges

[13.1]

Plot the densities of the major moons (say of size 100 km and up) against their distances from the planet for each of the giant planets. Place on the plot the synchronous orbit limit for each planet. From the results of this plot, what can you deduce about the origins of the moons of the outer planets? The synchronous radius has importance for accelerating satellites into higher and lower orbits. Illustrate how this happens. Make a copy of your plot for Q. [13.2]. [Hint: spreadsheets are very useful for this type of work.]

[13.2]

Calculate the Roche limits for density values in the range 1,000–3,000 kg/m3 and place these along with the observed limits for the ring systems on a copy of your plot for Q. [13.1]. Now what can you deduce about the origin of the ring systems?

[13.3]

The Galilean satellites have been shown to have orbital resonances among them (see Wilson and Milone 2014, Sect. 3.7.3). Examine closely the periods of the other giant planet moons for similar effects.

[13.4]

Organize the satellites of the outer planets in groups as suggested (a) by orbital characteristics and (b) by physical characteristics. Comment on the differences and similarities between the two groups and what this suggests for the origins of the moons in each group.

[13.5]

Consider the situation and conclusions of Sect. 13.2 for the case of a retrograde satellite.

[13.6]

The heights of the eruptive plumes on Io vary from 60 to 400 km. Given the mass and radius of Io from Table 13.2, compute the ejection velocities required to reach these heights, and compare them to the escape velocity.

[13.7]

Compute the escape velocity from Titan, and the molecular weight of a molecule that can be retained over billions of years, assuming no major or prolonged temperature increases. Discuss the retention of NH3 and CH4 on Titan, and of volatile gases generally from the objects in the outer solar system.

Herschel, W.: ”Observations and Reports tending to the Discovery of one or more Rings of the Georgian Planet, and the flattening of its polar Regions,” In “On the Discovery of four additional Satellites of the Georgium Sidus. The retrograde Motion of its old Satellites announced; and the Cause of their Disappearance at certain Distances from the Planet explained.” Phil. Trans. Roy. Soc. London 88, 47–79 (esp., pp. 67–70) (1798)Google Scholar